Now, It loads only one advanced datagrid. How can I make the others ADG available after the form loads?

When I select an option in the comboBox it starts getting data and trying to fill the datagrids but I got an error "Cannot access a property or method of a null object reference" because the datagrid is not available.

There are various application frameworks designed to manage this for you. Fundamentally, the idea is that there is a central model and on creationComplete of the DataGrid, it accesses the model and assigns its own dataprovider.

I am application framework agnostic. I’ve never used one. I’ll probably get in trouble with the app framework folks, but IMHO, if you work for a company and they are only going to have one or two apps, I’d probably just use creationComplete. If you are a consultant banging out similar apps for several clients, then there is advantage to learning one so you can assemble apps out of re-usable pieces. App frameworks have a cost in performance and control over optimizations and are yet another thing to learn.